A phosphorescent phosphor is a phosphor which continues to emit fluorescence even after termination of excitation after some excitation is imparted to the phosphor to emit fluorescence. For a phosphorescent phosphor, along with diversification and high functionalization of display, multi-coloring, long afterglow and improved weather resistance of a phosphorescent phosphor are desired. With conventional phosphorescent phosphors, the types of colors of the fluorescence and afterglow were limited, the weather resistance was poor, and the afterglow time was short.
As a blue-emitting phosphorescent phosphor, a (Ca,Sr)S:Bi phosphor is known. As a yellowish green-emitting phosphorescent phosphor, a ZnS:Cu phosphor is known, and as a red-emitting phosphorescent phosphor, a (Zn,Cd)S:Cu phosphor is known.
However, the above-mentioned (Ca,Sr)S:Bi phosphor is not practically used at present, since the chemical stability of the host material is extremely poor, and the luminance and afterglow characteristics are inadequate. On the other hand, the (Zn,Cd)S:Cu phosphor is not practically used at present, since Cd which is a toxic substance, occupies almost a half of the host material, and the luminance and afterglow characteristics are not satisfactory. ZnS:Cu is also susceptible to decomposition by ultraviolet rays in the presence of moisture and is likely to be blackened, and the afterglow characteristics are also unsatisfactory, but it is inexpensive and is used mostly for face plates of clocks or for indoor use such as emergency escape signs.